[Paper Review] Abundance gradients and their evolution in the Milky Way disk

J. L. Hou, Nikos Prantzos+1 more|arXiv (Cornell University)|Jul 12, 2000

Stellar, planetary, and galactic studies1 references3 citations

TL;DR

This study uses a chemical evolution model of the Milky Way disk to reproduce observed abundance gradients for 12 elements (He to Zn), showing that massive star yields—especially M92 yields—can explain carbon production without intermediate-mass stars, while nitrogen requires additional sources like intermediate-mass stars. The model predicts a flattening of abundance gradients over time due to an 'inside-out' formation scenario, with lower abundance scatter in the inner disk than outer regions.

ABSTRACT

Based on a simple, but fairly successful, model of the chemical evolution of the Milky Way disk, we study the evolution of the abundances of the elements He, C, N, O, Ne, Mg, Al, Si, S, Ar and Fe. We use metallicity dependent yields for massive stars with and without mass loss. We find that most observed abundance profiles are correctly reproduced by massive star yields, but C and N require supplementary sources. We argue that massive, mass losing stars can totally account for the abundance profile of C, while intermediate mass stars are the main source of N; in both cases, some conflict with corresponding data on extragalactic HII regions arises, at least if current observations in the Galaxy are taken at face value. The observed behaviour of Al is marginally compatible with current massive star yields, which probably overestimate the ``odd-even'' effect. We also find that the adopted ``inside-out'' formation scheme for the Milky Way disk produce abundance profiles steeper in the past. The corresponding abundance scatter is smaller in the inner disk than in the outer regions for a given interval of Galactic age.

Motivation & Objective

• To test whether current massive star yields can reproduce observed abundance gradients in the Milky Way disk across multiple elements.
• To investigate the role of intermediate-mass stars in nitrogen production, given discrepancies with extragalactic HII region trends.
• To examine the evolution of abundance gradients over time, particularly whether they were steeper in the past.
• To assess the impact of the 'inside-out' formation scenario on abundance scatter and gradient steepness.
• To evaluate the compatibility of current yields with observed abundance ratios (e.g., C/O, N/O) and resolve tensions with extragalactic data.

Proposed method

• The study employs a chemical evolution model (BP99) with radial variations in infall and star formation timescales to simulate disk evolution over 13.5 Gyr.
• It incorporates metallicity-dependent yields from massive stars with and without mass loss (specifically M92 and WW95 yields) for elements from He to Zn.
• The model assumes an 'inside-out' formation scheme, where star formation begins in the inner disk and progresses outward over time.
• Abundance gradients are computed as dlog(X/H)/dR, comparing theoretical profiles with observational data from HII regions, B-stars, and planetary nebulae.
• The model evaluates abundance ratios (e.g., C/O, N/O, Ne/O) as functions of galactocentric radius to test nucleosynthetic origin predictions.
• The abundance scatter is analyzed across different galactocentric radii to test the model’s prediction of lower scatter in the inner disk due to age dispersion.

Experimental results

Research questions

• RQ1 Can current massive star yields reproduce the observed abundance gradients for O, C, N, Ne, Mg, Al, Si, S, Ar, and Fe in the Milky Way disk?
• RQ2 Why do observed C and N abundance profiles require additional nucleosynthetic sources beyond massive stars?
• RQ3 Is the observed trend of increasing C/O and N/O with metallicity in extragalactic HII regions consistent with Milky Way data?
• RQ4 Did abundance gradients in the Milky Way disk become flatter over time, and does the 'inside-out' formation model explain this evolution?
• RQ5 Does the observed abundance scatter in planetary nebulae support the model’s prediction of lower scatter in the inner disk?

Key findings

• The model reproduces the observed oxygen abundance gradient of dlog(O/H)/dR ≈ -0.06 dex kpc⁻¹, consistent with observations of young objects.
• Massive star yields (M92) can fully account for carbon production across the disk, eliminating the need for intermediate-mass stars as the primary source.
• Nitrogen production requires an additional source, most plausibly intermediate-mass stars, due to inconsistencies between observed and predicted N/O gradients.
• The model predicts a steady flattening of abundance gradients over time due to the 'inside-out' formation scenario, with steeper gradients in the past.
• Abundance scatter is predicted to be smaller in the inner disk than in the outer regions for a given age interval, a testable prediction based on planetary nebulae data.
• The model shows that Al abundance is marginally compatible with current massive star yields, though these may overestimate the 'odd-even' effect in nucleosynthesis.